times as great, his content is
1390 times as great. Now even (supposing we assume like temperatures and
like densities) if the only effect were that through a given area of
surface eleven times more matter had to be cooled in the one case than
in the other, there would be a vast difference between the times
occupied in concentration. But, in virtue of a second factor, the
difference would be much greater than that consequent on these
geometrical relations. The escape of heat from a cooling mass is
effected by conduction, or by convection, or by both. In a solid it is
wholly by conduction; in a liquid or gas the chief part is played by
convection--by circulating currents which continually transpose the
hotter and cooler parts. Now in fluid spheroids--gaseous, or liquid, or
mixed--increasing size entails an increasing obstacle to cooling,
consequent on the increasing distances to be travelled by the
circulating currents. Of course the relation is not a simple one: the
velocities of the currents will be unlike. It is manifest, however, that
in a sphere of eleven times the diameter, the transit of matter from
centre to surface and back from surface to centre, will take a much
longer time; even if its movement is unrestrained. But its movement is,
in such cases as we are considering, greatly restrained. In a rotating
spheroid there come into play retarding forces augmenting with the
velocity of rotation. In such a spheroid the respective portions of
matter (supposing them equal in their angular velocities round the axis,
which they will tend more and more to become as the density increases),
must vary in their absolute velocities according to their distances from
the axis; and each portion cannot have its distance from the axis
changed by circulating currents, which it must continually be, without
loss or gain in its quantity of motion: through the medium of fluid
friction, force must be expended, now in increasing its motion and now
in retarding its motion. Hence, when the larger spheroid has also a
higher velocity of rotation, the relative slowness of the circulating
currents, and the consequent retardation of cooling, must be much
greater than is implied by the extra distances to be travelled.

And now observe the correspondence between inference and fact. In the
first place, if we compare the group of the great planets, Jupiter,
Saturn, and Uranus, with the group of the small planets, Mars, Earth,
Venus, and Mercury, we